Polyesters such as, for example, polyethylene terephthalate, polytrimethylene terephthalate and polybutylene terephthalate, generally referred to as "polyalkylene terephthalates", are a class of important industrial polymers. They are widely used in thermoplastic fibers, films, and molding applications.
Polyalkylene terephthalates can be produced by transesterification of a dialkyl terephthalate ester with a glycol followed by polycondensation or by direct esterification of terephthalic acid with the selected glycol followed by polycondensation. A catalyst is used to catalyze the esterification, transesterification or polycondensation.
Antimony, in the form of a glycol solution of antimony oxide, frequently is used as catalyst in the transesterification or esterification process. However, antimony forms insoluble antimony complexes that plug fiber spinnerets and leads in fiber spinning to frequent shutdowns to wipe spinnerets clean of precipitated antimony compounds. The antimony-based catalysts are also coming under increased environmental pressure and regulatory control, especially in food contact applications.
Organic titanates, such as tetraisopropyl and tetra n-butyl titanates, are known to be effective polycondensation catalysts for producing polyalkylene terephthalates in general, and frequently are the catalyst of choice. However, these catalysts tend to hydrolyze on contact with water forming glycol-insoluble oligomeric species, which lose catalytic activity. These organic titanates also generate a significant amount of yellow discoloration when used as polyesterification catalysts. Additionally, many organic titanate catalysts are also substantially insoluble in a polymerization mixture thereby creating non-uniform distribution of catalyst in the mixture.
Water compatible titanates, such as titanium bis-ammonium lactate, bis-triethanolamine titanate or the titanium sodium citrate catalysts disclosed in EP 812818, when used as polyesterification catalysts, generate significant yellow discoloration in the resultant polymer. Similarly, WO 99/28033 discloses an organometallic compound for producing an ester. The organometallic compound comprises the reaction product of an orthoester of titanium, zirconium, or aluminum, an alcohol containing at least two hydroxyl groups, an organophosphorus compound, and a base. When used as polyesterification catalyst, however, it was found that the organometallic compound also generates undesirably significant yellow discoloration in the final product.
Therefore, there is an increasing need for the development of a new catalyst that is substantially soluble in a glycol, is efficient, produces a polymer with reduced color, exhibits good catalytic activity, does not result in plugging fiber spinnerets, and is environmentally friendly.
An advantage of the present invention catalyst composition is that the polymer produced using the invention catalyst has improved optical properties (e.g., less undesirable color) compared to polymer produced using an organic titanate catalyst alone. Another advantage of the present invention is that the invention catalyst is soluble in an alcohol used to produce a polyester, does not generate precipitates in the polyester that plug fiber spinnerets, and is environmentally friendly. Other advantages will become more apparent as the invention is more fully disclosed herein below.